private Loader obtainInstance(String documentId) throws FileNotFoundException {
   Loader loader;
   synchronized (mArchives) {
     loader = getInstanceUncheckedLocked(documentId);
     loader.getReadLock().lock();
   }
   return loader;
 }
 private void releaseInstance(@Nullable Loader loader) {
   if (loader != null) {
     loader.getReadLock().unlock();
   }
 }